Most commercial electrophotographic color processes form separate toner images on the same or separate members and transfer them in registration. With these processes, both exposure and transfer must be properly timed for good color registration.
LED printheads and other similar electronic exposure devices expose each line of an image at essentially one time in response to a timing signal. U.S. Pat. No. 4,821,066 Foote et al describes a color printer in which this timing signal is generated in response to a set of perforations along the edge of a web image member. The perforations are sensed by a printhead sprocket, which printhead sprocket drives a rotary encoder. Because each line or set of lines is exposed in response to an encoder signal, the exposure is independent of variations in the speed of the receiver. Variations in the speed of moving webs are particularly difficult to eliminate and, without an encoder, defects caused by such variation will be noticeable in a high resolution final image.
In the Foote et al patent single color images are both exposed and transferred in response to sprocket engagement of the same perforation. Two or more images are superimposed in transfer using sprocket-perforation timing based on the encoder controlled LED exposure. Inaccurate positioning of lines of exposure that are ultimately superimposed will alter the color of that line.
U.S. patent application Ser. No. 232,073, Mosehauer et al (corresponding WIPO publication 8903400, 1990), describes an electrophotographic process in which a multicolor image is formed using two or more electronic exposures of a single frame of an image member. See also, U.S. Pat. Nos. 4,669,864; 4,819,028 and 4,540,272. Transfer of the multicolor image is in one step and does not affect registration of colors. In this process, registration of the exposures is critical to final image quality. Again, exposure timing is controlled by an encoder-sprocket-perforation system.
U.S. Pat. No. 4,837,636 discloses printing apparatus in which a row of marks along the edge of a receiver cooperates with a light source and a CCD series for sensing the velocity of a recording member in copying/printing apparatus. This sensed velocity is fed back to the printer to control the printer drive mechanism.
Other patents of possible interest are U.S. Pat. Nos. 3,914,047; 4,505,576; 4,518,862; 4,803,515; 4,607,950; 4,734,788; 4,752,804; EP O 319 241 A2; and European Patent Application 0 291 738.